Reducing the pipe dimension to a heating device

Let's say there is a water based heating device that has 3/4" connection nipples. The pipe that leads to the device is 5/4". If one ignores pressure- and head loss, does it make any difference how far from the device the pipe reduces from 5/4" to 3/4" ?

The following is not a very technical explanation - and involves a gross oversimplification of the way that fluid flows through pipes - but it may help:

The inner surface of the pipe presents resistance to the water flowing through it - tending to slow it down. That resistance is along its entire length, so a 10m long pipe has twice the resistance of a 5m long pipe.

A pipe's circumference - where the pipe is touching the water - is proportional to the diameter, but it's cross-sectional area - the volume for the water to flow through - is proportional to the square of it's diameter.

Finally, the resistance of the pipe to the flow of water increases with the rate of flow.

For simplicity, if we assume that one pipe is double the diameter of the other, then:

If we double the pipe diameter for any given length, the area of the inside surface doubles but the volume quadruples - so overall the ratio of volume/surface area doubles. Thus, in a larger pipe, less of the total water in the pipe is in contact with the frictional surface so the water can flow more freely.
Not only that, but for a given flow rate of water, a big pipe means the water can travel more slowly, so the resistance to flow caused by the surface of the pipe is reduced.

To relate it to your question, the increased resistance of the narrower pipe (compared to a wide pipe) will act on the water all along its length, and will reduce flow rates.

1) you have to buy more 3/4 inch pipe and less 5/4 inch pipe
2) you have to buy a reducer (5/4 to 3/4) and a 3/4 fitting to the device (instead of just a reducing fitting)
3) If the pipe also supports the device, there may be unacceptable loads on the small pipe if it is too long or if it is horizontal
4) you may not be able to access the 5/4 to 3/4 reducer to fix it when it leaks
5) it may not look as nice
6)...

sorry, but your question struck me as hopelessly vague (what does "does it make any difference" mean??). also, why would you want to "ignore pressure and head loss"?

I agree with gmax. If you ignore pressure/head losses, then no, there shouldn't be any difference. However, as previously posted, if the pipe length is large, then reducing the size of the pipe could have an effect on the pump needed to get the water there.

I'm not going to ignore it, I'm only saying that I'm aware of its effect on flow, thus I need no one telling me what they are. What I need to know is simply if there are other factors - strictly flow related - that need to be considered?